Magnetic resonance (MR) techniques can be used to determine properties of a substance. One example of a MR technique is a nuclear magnetic resonance (NMR) measurement. A NMR measurement typically includes applying a static magnetic field to the substance. The static magnetic field generates an initial magnetization of atomic nuclei within the substance. Then, an NMR system is used to apply an oscillating magnetic field at a particular frequency to the substance. The oscillating field is composed of a sequence of pulses that tip the magnetization of the atomic nuclei away from the initial magnetization. The pulse sequence can be arranged so that pulses and the static field interact with the nuclei to produce an NMR signal composed of “echoes” from within at least a portion of the substance. The NMR signal is detected and then used to determine NMR properties such as T1 relaxation time, T2 relaxation time, and attenuation of the signal due to molecular diffusion. These NMR properties can be used to determine the properties of the substance within the shell.
The portion of the substance where the NMR signal is generated is known as a “shell” or a “slice.” “Non-resonant” NMR transmitters are able to apply pulse sequences at different frequencies to the substance and can investigate multiple shells within the substance in close temporal proximity. In turn, each shell produces an NMR signal at a different frequency. These signals can be received and then used to determine NMR properties of the shells.